In recent years, with an aim toward effective utilization of energy for greater environmental conservation and reduced usage of resources, a great deal of attention is being directed to power smoothing systems for wind power generation or overnight charging electric power storage systems, household dispersive power storage systems based on solar power generation technology, and power storage systems for electric vehicles and the like.
The number one requirement for cells used in such power storage systems is high energy density. The development of lithium ion batteries is therefore advancing at a rapid pace, as an effective strategy for cells with high energy density that can meet this requirement.
The second requirement is a high output characteristic. For example, in a combination of a high efficiency engine and a power storage system (such as in a hybrid electric vehicle), or a combination of a fuel cell and a power storage system (such as in a fuel cell electric vehicle), the power storage system must exhibit a high output discharge characteristic during acceleration.
Electrical double layer capacitors and nickel hydrogen cells are currently under development as high output power storage devices.
Electrical double layer capacitors that employ activated carbon in the electrodes have output characteristics of about 0.5 to 1 kW/L. Such electrical double layer capacitors not only have a high output characteristic but also high durability (especially cycle characteristics and high-temperature storage characteristics) and have been considered optimal devices for fields requiring the high output mentioned above. However, their energy densities are no greater than about 1 to 5 Wh/L. A need therefore exists for even higher energy density.
On the other hand, nickel hydrogen batteries commonly employed in existing hybrid electric vehicles exhibit high output equivalent to electrical double layer capacitors, and have energy densities of about 160 Wh/L. Still, research is being actively pursued toward further increasing their energy density and output characteristic and increasing their durability (especially stability at elevated temperatures).
Research is also advancing toward increased outputs for lithium ion batteries as well. For example, lithium ion batteries are being developed that yield high output exceeding 3 kW/L at 50% depth of discharge (the percentage (%) of discharge with respect to the service capacity of a power storage element). However, the energy density is 100 Wh/L or lower, and the design is such that the high energy density, which is the major feature of a lithium ion battery, is reduced. The durability (cycle characteristic and high-temperature storage characteristic) is inferior to that of an electrical double layer capacitor. In order to provide practical durability for such a lithium ion battery, therefore, they are used with a depth of discharge in a narrower range than 0 to 100%. Because the usable capacity is even lower, research is actively being pursued toward further increasing durability.
There is a strong demand for implementation of power storage elements exhibiting high energy density, high output characteristics and high durability, as mentioned above. Nevertheless, the existing power storage elements mentioned above have their advantages and disadvantages. New power storage elements are therefore desired that can meet these technical requirements. Promising candidates are power storage elements known as lithium ion capacitors, which are being actively developed in recent years.
A lithium ion capacitor is a type of power storage element using a nonaqueous electrolytic solution comprising a lithium salt (hereunder also referred to as “nonaqueous lithium power storage element”), wherein charge/discharge is accomplished by non-Faraday reaction by adsorption and desorption of anions similar to an electrical double layer capacitor at about 3 V or higher, at the positive electrode, and Faraday reaction by intercalation and release of lithium ions similar to a lithium ion battery, at the negative electrode.
To summarize the electrode materials commonly used in power storage elements, and their characteristics: usually, when charge/−discharge is carried out using a material such as activated carbon as an electrode, by adsorption and desorption of ions on the activated carbon surface (non-Faraday reaction), it is possible to obtain high output and high durability, but with lower energy density (for example, 1×). On the other hand, when charge/discharge is carried out by Faraday reaction using an oxide or carbon material as the electrode, the energy density is higher (for example, 10 times that of non-Faraday reaction using activated carbon), but then durability and output characteristic become issues.
Electrical double layer capacitors that combine these electrode materials employ activated carbon as the positive electrode and negative electrode (energy density: one-fold) and carry out charge/discharge by non-Faraday reaction at both the positive and negative electrodes and are therefore characterized by having high output and high durability, but also low energy density (positive electrode: one-fold×negative electrode: one-fold=1).
Lithium ion secondary batteries use a lithium transition metal oxide for the positive electrode (energy density: 10-fold) and a carbon material (energy density: 10-fold) for the negative electrode), carrying out charge/discharge by Faraday reaction at both the positive and negative electrodes, and therefore have high energy density (positive electrode: 10-fold×negative electrode: 10-fold=100), but have issues in terms of output characteristic and durability. In addition, the depth of discharge must be restricted in order to satisfy the high durability required for hybrid electric vehicles, and with lithium ion secondary batteries only 10 to 50% of the energy can be utilized.
A lithium ion capacitor is a type of asymmetric capacitor that employs activated carbon (energy density: one-fold) for the positive electrode and a carbon material (energy density: 10-fold) for the negative electrode, and it is characterized by carrying out charge/discharge by non-Faraday reaction at the positive electrode and Faraday reaction at the negative electrode and having the characteristics of both an electrical double layer capacitor and a lithium ion secondary battery. Moreover, a lithium ion capacitor exhibits high output and high durability, while also having high energy density (positive electrode: one-fold×negative electrode: 10-fold=10) and requiring no restrictions on depth of discharge as with a lithium ion secondary battery.
Various researches have been conducted in regard to positive electrodes for the power storage elements mentioned above (especially lithium ion secondary batteries).
In PTL 1 there is proposed a lithium ion secondary battery using a positive electrode containing lithium carbonate in the positive electrode and having a current shielding mechanism that operates in response to increased internal pressure in the battery. In PTL 2 there is proposed a lithium ion secondary battery employing a lithium complex oxide such as lithium manganate as the positive electrode, and with reduced elution of manganese by including lithium carbonate in the positive electrode. In PTL 3 there is proposed a method of causing restoration of the capacitance of a deteriorated power storage element by oxidizing various lithium compounds as coated oxides at the positive electrode. However, the positive electrodes described in PTLs 1 to 3 still have room for improvement in terms of their application to asymmetric capacitors such as lithium ion capacitors, and absolutely no research has been conducted on their suppression of decomposition of lithium compounds in the positive electrode and suppression of gas generation and increased resistance, particularly during high-load charge/discharge cycling of nonaqueous lithium power storage elements. Moreover, the electrodes described in PTLs 1 to 3 still have room for improvement in terms of their application to nonaqueous power storage elements including electrode laminated bodies or wound electrodes that have complex multilayer structures, and in particular, absolutely no research has been conducted on suppressing all of thermal runaway during internal short circuiting of nonaqueous lithium power storage elements, increased resistance in high-load charge/discharge cycling, and gas generation in high-temperature, high-voltage environments. PTL 4 proposes a lithium ion capacitor having low deviation in thickness of the electrode layer on the front and back sides. However, suppression of thermal runaway during internal short circuiting has not been considered in any way with the electrode described in PTL 4.
On the other hand, PTL 5 proposes a power storage element employing activated carbon as the positive electrode active material, and as the negative electrode active material, a carbonaceous material obtained by intercalating lithium by a chemical process or electrochemical process in a carbon material capable of intercalating and releasing lithium in an ionized state. In PTL 5, the carbon materials mentioned are natural graphite, artificial graphite, graphitized mesophase carbon microspheres, graphitized mesophase carbon fibers, graphite whiskers, graphitized carbon fibers, thermal decomposition products of furfuryl alcohol resin or novolac resin, and thermal decomposition products of polycyclic hydrocarbon condensation polymer compounds such as pitch coke.
PTLs 6 to 10 each propose electrodes and a power storage element using activated carbon as the positive electrode active material and using as the negative electrode active material a composite porous material with a carbonaceous material covering the surface of activated carbon, where the negative electrode active material has been doped with lithium in a predetermined amount. The lithium ion capacitors using these negative electrode active materials have low internal resistance compared to lithium ion capacitors using other materials such as graphite for the negative electrode active material, and therefore high input/output characteristics are obtained.
The purposes for which lithium ion capacitors are used include power storage elements for railways, construction machines and automobiles, for example. Further improvement in energy density is being sought for such purposes, while still maintaining a high input/output characteristic and a high load charge/discharge cycle characteristic.
One method for increasing the energy density is to lower the thickness of the negative electrode active material layer to reduce the cell volume, while maintaining the same energy. With decreasing thicknesses of negative electrode active material layers, the weight of the negative electrode active material per unit area of the negative electrode decreases, and therefore the utilizable capacity per unit weight of the negative electrode active material during charge/discharge of the lithium ion capacitor increases. In other words, when the negative electrode active material layer is to be formed as a thin-film it is preferred to use a negative electrode material with a large reversible capacitance. Such negative electrode materials include alloy-type negative electrode materials such as silicon, silicon oxide and tin, that form alloys with lithium.
NPL 1 discloses a lithium ion capacitor employing activated carbon as the positive electrode active material and silicon as the negative electrode active material. However, investigation by the present inventors has demonstrated that when such lithium ion capacitors are used for charge/discharge cycling several times with a very high current (hereunder also referred to as “high-load charge/discharge cycling”), the capacitance markedly decreases, and that the tendency is more notable the smaller the film thickness of the negative electrode active material layer.